Oxo-anions such as nitrate and chromate are pollutants that are harmful in drinking water. Nitrate is a prevalent groundwater pollutant, and is regulated in drinking water. Chromium is also a known drinking water contaminant that poses significant human health risks. Both hexavalent and trivalent chromium forms have been linked to adverse health effects. These findings have raised a concern among the general public and spurred a campaign to regulate by decreasing Cr(VI) levels in drinking water.
Nitrate removal from contaminated waters is difficult with conventional water treatment methods because of nitrate's unique chemistry. Among available nitrate removal technologies, ion exchange (IX) is used frequently in drinking water applications because of its effectiveness, low cost, and operational simplicity in producing reliable drinking water. The ion exchange media used for nitrate removal include non-selective resins and nitrate-to-sulfate selective resins (e.g., Purolite A-520E). But the IX process generates large volumes of waste brine containing nitrate, chloride, sulfate, and other oxyanions. Disposal of IX brine has become a costly challenge from economic and environmental standpoints.
Various other technologies aim to convert nitrate into harmless nitrogen gas (N2); these include biological denitrification, catalytic reduction, and electrochemical reduction. Biological denitrification has not been widely adopted for drinking water applications because of its high capital costs, lengthy lead times for biofilm establishment, and post-treatment requirements for the removal of biomass and dissolved organics. Other reduction technologies also have serious disadvantages (e.g., poor selectivity to nitrogen, hydrogen availability, energy intensiveness) that limit their practical applications and will not soon replace IX in drinking water treatment.
Integration of IX with brine denitrification processes is promising because the combination could reduce IX costs for brine disposal and minimize problems associated with each denitrification approach for drinking water treatment. Biological denitrification to remove nitrate from IX brine has been studied. By treating IX brine instead of the drinking water itself, quality concerns, such as bacterial, organic matter, and hazardous by-product contamination caused by bioprocesses, are reduced. However, IX systems are often operated intermittently (i.e., a few times per month to meet peak water demands), which makes inclusion of biological processes challenging. Electrochemical reduction can selectively treat nitrate in high salt solutions, but may produce Cl2, which can damage the structure of IX resins. An attempt to reduce nitrate using zerovalent iron nanoparticles showed that the nitrate removal rate was greatly slowed in 6% NaCl solution, and ammonium was the predominant by-product accounting for 62% of the reduced nitrate. An effective nitrate reduction technology for IX brine treatment would overcome these disadvantages.
Treatment options for Cr(VI) have traditionally fallen into six treatment categories, including coagulation-precipitation-filtration, adsorption to different media, ion exchange, membrane technology, electrodialysis, and biological removal. These technologies are often troubled by disadvantages stemming from challenges associated with cost, scalability, and reliability to achieve low Cr(VI) concentrations. While studies have demonstrated that uniquely synthesized and modified semiconductor UV/VIS photocatalysts are capable of reducing and removing hexavalent chromium from water to acceptable levels, much of the work focuses on laboratory scale conditions and commercially unavailable photocatalysts.